Add new PNG images for clean and preview outputs

- Added `image_clean.png` to the output directory for the clean image representation.
- Added `image_clean_preview.png` for the preview of the clean image.
- Introduced `image_svg_clean.png` for the SVG clean image representation.

engine/unit_manager.py

@@ -5,6 +5,20 @@ from units import gas, rat, bomb, mine
 
 
 class UnitManager:
+    def _spawnable_rat_positions(self):
+        positions = []
+        for y in range(1, self.map.height - 1):
+            for x in range(1, self.map.width - 1):
+                if not self.map.is_empty(x, y):
+                    continue
+                for dx, dy in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
+                    nx = x + dx
+                    ny = y + dy
+                    if self.map.in_bounds(nx, ny) and self.map.is_empty(nx, ny):
+                        positions.append((x, y))
+                        break
+        return positions
+
     def has_weapon_at(self, position):
         """Check if there's a weapon (bomb, gas, mine) at the given position"""
         for unit in self.units.values():
@@ -13,6 +27,16 @@ class UnitManager:
                 if isinstance(unit, (bomb.Timer, bomb.NuclearBomb, gas.Gas, mine.Mine)):
                     return True
         return False
+
+    def can_place_weapon_at(self, position):
+        x, y = position
+        if not self.map.in_bounds(x, y):
+            return False
+        if not self.map.is_empty(x, y):
+            return False
+        if self.has_weapon_at(position):
+            return False
+        return True
         
     def count_rats(self):
         count = 0
@@ -22,7 +46,7 @@ class UnitManager:
         return count
 
     def spawn_gas(self, parent_id=None):
-        if self.map.is_wall(self.pointer[0], self.pointer[1]):
+        if not self.can_place_weapon_at(self.pointer):
             return
         if self.ammo["gas"]["count"] <= 0:
             return
@@ -33,23 +57,33 @@ class UnitManager:
     def spawn_rat(self, position=None):
         if position is None:
             position = self.choose_start()
+        if position is None:
+            print("[flow] spawn_rat aborted: no valid spawn position", flush=True)
+            return
+
+        print(f"[flow] spawn_rat using position={position}", flush=True)
         
         # Don't spawn rats on top of weapons
         if self.has_weapon_at(position):
             # Try nearby positions
             for dx, dy in [(0,1), (1,0), (0,-1), (-1,0), (1,1), (-1,-1), (1,-1), (-1,1)]:
                 alt_pos = (position[0] + dx, position[1] + dy)
-                if not self.map.is_wall(alt_pos[0], alt_pos[1]) and not self.has_weapon_at(alt_pos):
+                if not self.map.in_bounds(alt_pos[0], alt_pos[1]):
+                    continue
+                if self.map.is_empty(alt_pos[0], alt_pos[1]) and not self.has_weapon_at(alt_pos):
                     position = alt_pos
                     break
             else:
                 # All nearby positions blocked, abort spawn
+                print(f"[flow] spawn_rat aborted: weapon blocks spawn near {position}", flush=True)
                 return
         
         rat_class = rat.Male if random.random() < 0.5 else rat.Female
         self.spawn_unit(rat_class, position)
         
     def spawn_bomb(self, position):
+        if not self.can_place_weapon_at(position):
+            return
         if self.ammo["bomb"]["count"] <= 0:
             return
         self.render_engine.play_sound("PUTDOWN.WAV")
@@ -60,7 +94,7 @@ class UnitManager:
         """Spawn a nuclear bomb at the specified position"""
         if self.ammo["nuclear"]["count"] <= 0:
             return
-        if self.map.is_wall(position[0], position[1]):
+        if not self.can_place_weapon_at(position):
             return
         self.render_engine.play_sound("NUCLEAR.WAV")
         self.ammo["nuclear"]["count"] -= 1
@@ -69,7 +103,7 @@ class UnitManager:
     def spawn_mine(self, position):
         if self.ammo["mine"]["count"] <= 0:
             return
-        if self.map.is_wall(position[0], position[1]):
+        if not self.can_place_weapon_at(position):
             return
         self.render_engine.play_sound("PUTDOWN.WAV")
         self.ammo["mine"]["count"] -= 1
@@ -83,12 +117,11 @@ class UnitManager:
             self.units[id] = unit(self, position, id, **kwargs)
 
     def choose_start(self):
-        if not hasattr(self, '_valid_positions'):
-            self._valid_positions = [
-                (x, y) for y in range(1, self.map.height-1) 
-                for x in range(1, self.map.width-1) 
-                if self.map.matrix[y][x]
-            ]
+        if not hasattr(self, '_valid_positions') or self._valid_positions is None:
+            self._valid_positions = self._spawnable_rat_positions()
+            print(f"[flow] choose_start computed {len(self._valid_positions)} spawnable cells", flush=True)
+        if not self._valid_positions:
+            return None
         return random.choice(self._valid_positions)
     
     def get_unit_by_id(self, id):